Twenty-one relatives of patients with documented hypertrophic cardiomyopathy were studied prospectively over a period of 5 years with M-mode and wide-angle two-dimensional echocardiography to evaluate the potential for progression of left ventricular hypertrophy. Although each of these subjects had no or minimal hypertrophy prior to adolescence, 11 showed marked increase in the magnitude and distribution of hypertrophy including 3 who progressed from a normal heart. These findings also emphasize that echocardiographic screening of pre-adolescent family members cannot definitively exclude hypertrophic cardiomyopathy.